Due to relatively high instances of system inertia and delay in automotive transmissions, exclusively using feedback control of various components in automotive transmissions may not be sufficient for certain transient schemes. In such transient cases, feedforward control may be used to anticipate system changes. For example, mixed feedforward and feedback control can be used for a smooth upshift in an automatic transmission without causing significant “feel” issues for the driver, thereby improving overall shift quality.
An upshift operation in an automatic transmission typically involves a first clutch (or releasing element) that disengages from a first gear while a second clutch (or applying element) engages a second gear. A third clutch remains engaged during the upshift operation. Therefore, at least two clutches are typically engaged during each gear level of an automatic transmission. For example, an upshift in a 4-speed automatic transmission may be a 1-2 shift, a 2-3 shift, and/or a 3-4 shift.
Referring now to FIG. 1, clutches in automatic transmissions are typically actuated by pressure from fluid such as oil. Oil fills a cavity including a piston and displaces the piston to engage the clutch. Automatic transmission systems also typically include accumulators 10 for each clutch. As oil fills the cavity including the clutch it also fills the accumulator 10. The accumulator 10 includes a spring 12 that is compressed when a piston 14 in the accumulator 10 is displaced by pressure from the oil. The oil begins to displace the piston 14 after all of the empty space in the accumulator 10 is filled with oil and the pressure of the oil increases. Therefore, accumulators 10 reduce the sensitivity of torque variations during the torque phase of an upshift.
However, a solenoid controls the flow rate of oil to the accumulator 10 and an associated clutch. The duty cycle of a control signal applied to the solenoid determines the fraction of time during which the solenoid valve is open or closed. Since the solenoid controls the flow rate and ultimately the pressure of the oil, the accumulator 10 makes the pressure response slower and more difficult to predict. The calculated volume of oil present in the accumulator 10 at a given time is an indicator of the capacity of the clutch at that time. Therefore, the calculated volume can be used for control purposes. However, since the oil compresses the spring 12 in the accumulator 10, the volume of the accumulator 10 varies over a usable range. Therefore, the calculated volume may not remain consistent throughout the life of the transmission. Additionally, there is no conventional way to directly measure the volume of the oil at a given time.